Low-Voltage, Silicon RF Power

Amplifier/Predriver

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7

An overall loaded Q

≤ 5 can be achieved with readily

available surface-mount components. This network

absorbs the parasitic elements of the surface-mount

components in such a way that they do not negatively

impact the stopband characteristics; in fact, they can

improve the overall stopband attenuation with properly

chosen components. High-Q components (Q > 100)

that have self-resonance near the 3rd harmonic of the

intended output frequency should provide good pass-

band characteristics with low loss, while offering good

attenuation of the undesired 2nd and 3rd harmonics

that are generated. Note that most applications will

require extra filtering components and good shielding

after the matching network to ensure absolute attenua-

tion of out-of-band signals in order to meet out-of-band

spurious suppression requirements.

Output Mismatch Considerations

The MAX2430 will typically withstand an output load

mismatch of VSWR = 6:1 at any electrical phase without

exhibiting oscillatory behavior over the entire supply

bility under load mismatch conditions and does not

affect normal operation of the circuit.

BIAS Pin

The voltage at the BIAS pin controls the output power

transistor biasing. At BIAS = 0.6V, the output transistor

is biased to Class C, resulting in low gain and relatively

nonlinear power. Above 2V, the output stage is biased

to Class AB. Note that changing the bias voltage may

degrade the output transistor’s stability.

The shutdown pin (SHDN) controls the master bias cir-

cuit, which in turn provides a control current of approxi-

mately ±500µA to the external capacitor connected to

the BIAS pin. When SHDN transitions from low to high,

the BIAS pin capacitor charges up and clamps at

approximately 2.2V. When SHDN transitions from high

to low, the BIAS pin capacitor is discharged to nearly

ground. This results in a power-up/power-down ramp-

ing of the RF envelope, which can be approximated by

the following equation:

Therefore, a 2.2nF capacitor will give approximately

10µs ramp time.

The BIAS pin can also be used to control the final output

power and gain over a 15dB range, by forcing the BIAS

pin voltage externally between 0.6V and 2.4V. Note that

the BIAS pin driver must be able to source/sink 700µA.

Forcing the BIAS pin directly in this manner disrupts the

RF envelope timing function. To avoid this, place a

diode in series with the BIAS pin control circuit, as

shown in Figure 2.

Note that when using the BIAS pin for power control,

linearity is much degraded at the lower power levels.

Operating Frequency Range

The MAX2430 has been characterized for operation in

the 800MHz to 1000MHz range. Operation outside this

range is possible, but the following issues must be con-

sidered:

• Gain increases substantially at lower frequencies,

possibly causing stability problems.

• Useful gain and output power levels drop rapidly

above 1000MHz.

MAX2430

BIAS

10

0V TO 2.0V

POWER

CONTROL

2

SHDN

MASTER

BIAS

2.2V

CLAMP

OUTPUT

BIAS

Figure 2. Power-Control Application Using BIAS Pin